15
votes
How does IQ modulation work (intuitively)?
I think it's more intuitive if you unlearn some things first.
Oscillation is not:
$$ \cos(\omega t) $$
where $\omega$ is the angular frequency in radians per second, and $t$ is time.
Rather, ...
- 52.5k
12
votes
Accepted
How does signal strength relate to bandwidth?
It depends on what you mean by "stronger".
Ignoring actual limitations of your hardware and just considering the theory of communications, if you have 25 W of transmit power, then you can spread that ...
- 25k
11
votes
Is FM effectively spread spectrum?
This is the sort of question that often comes down to “what definition of the word is useful” — not necessary any objective truth. However, I would be inclined to say that no, wider FM should not be ...
- 25k
7
votes
Accepted
How does a Faraday cage block an EMP?
Faraday cages block EMP in the same way they block all other time-varying electromagnetic fields. The only difference between blocking EMP versus blocking an ordinary radio transmission is the EMP is ...
- 52.5k
7
votes
Accepted
Key CW on zero crossings - zero bandwidth?
You raise an excellent question and your thought processes are indeed on the right track.
First some background. An ideal, uninterrupted sinusoidal carrier has zero bandwidth. Real world factors such ...
- 18.6k
6
votes
Can a common-mode current exist on the inside of a coax shield?
It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed.
Skin Effect
When direct current (time ...
- 18.6k
6
votes
Accepted
Why do we talk about common mode current from an antenna, but not from a transmitter?
The short answer is that it can't.
A shielded transmitter, connected to an ideal piece of coax, does not generate common mode currents.
The inner and the outer of the cable look connected (and for ...
- 12.2k
6
votes
Accepted
What does antenna frequency range mean in practice?
It means the range of frequencies in which the antenna is designed to operate. Operated outside that range, the antenna may not meet its specifications.
This particular antenna is a telescopic whip, a ...
- 52.5k
6
votes
Accepted
Why do PSK modes have "bandwidth"?
Binary PSK with instantaneous phase shifts would be equivalent to multiplying a sine wave (the carrier) with a square wave with values at 1 or -1.
When two signals are multiplied, this forms a ...
- 52.5k
6
votes
Accepted
What is the apparent AC voltage source caused by a mismatched impedance on the antenna or transmission line?
This would usually take a course in Transmission Line Theory, but we can give you the mechanical solution really quickly.
Behold: THE SMITH CHART (in impedance form):
Wow. A mandala! A fractal! What ...
- 17.4k
5
votes
How does IQ modulation work (intuitively)?
I believe a good point of view is the concept of orthogonality.
This is clear under everybody's eyes when seen in physical space, take for instance a 2-dimension space, a plane.
In the example above ...
- 183
5
votes
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Understanding how antenna mismatch can damage a transmitter
There are a few parts to this, it's good to keep them separate.
1. Antenna impedance
Antennas are complicated beasts, they connect the circuit world of voltage, current and power, into the ...
- 12.2k
5
votes
Accepted
Why are there weird sounds in radio?
What you describe is at least in part a feature of single side band demodulation. The radio is taking a range ("window" of bandwidth) of RF spectrum and shifting it to the audio spectrum. ...
- 6,960
4
votes
Key CW on zero crossings - zero bandwidth?
There is no way to transmit information in a signal with zero bandwidth. Switching the carrier at the zero crossings would reduce bandwidth but not take it to zero.
There's a mathematical explanation ...
- 52.5k
4
votes
Can you create a radio beam with a Faraday cage?
I think that if the sphere is very large in wavelengths. Maybe 1000 wl and if the opening is small, maybe 10x10 wl, you would create a fairly narrow beam.
Such a small opening in such a big sphere ...
- 1,019
4
votes
Accepted
Conventional or electron current
Current flow is electron flow and it is from negative to positive.
This is what we call class A hogwash.
Current notation is just a convention. Going by electron flow is not righter than going the ...
- 17.4k
4
votes
SDR sampling bandwidth - do the bits per sample matter?
For HF usage with 200 Msps converters and above, an 8 bit ADC will provide very good performance. You write: "the sampling frequency has to be several times the target frequency" but that is not true ...
- 1,019
4
votes
Accepted
Does the E-field propagate faster that the H-field?
The E and H fields of all RF signals travel at exactly the same speed - the speed of light. The phase relationship between the two fields remains constant as they travel through various mediums.
...
- 18.6k
4
votes
Accepted
Voltage wave in transmission lines
A transmission line is linear, which means we can consider the voltage at any point to be the superposition of two other voltages.
Asking what they are "physically" may or may not be a useful ...
- 52.5k
4
votes
Accepted
Transmitting Video Underwater
Here is some useful scholarly information about the signal attenuation (in dB) of EM waves through freshwater at various frequencies and depths. You will find a formula that you can use.
You will also ...
- 8,174
4
votes
What cause `multiply` has difference result between `float` and `complex` signal?
That's just the math behind it – everything is alright with these results!
You need to write down the formula of the real-valued $\sin(t)$ in terms of $e^{j2\pi t}$ and $e^{-j2\pi t}$, and you'll see ...
- 17.4k
4
votes
Accepted
What cause `multiply` has difference result between `float` and `complex` signal?
This is a consequence of what was discussed in your previous question, that any real-valued function like $\sin(\omega t)$ consists of both positive and negative frequencies.
Multiplying by a complex ...
- 52.5k
3
votes
How can the output power of a Yaesu FT101E be increased?
Should it be confirmed that the old finals are responsible for the low output then there could be only one solution i.e to replace the old finals with new tubes.
Thermionic emission from an old ...
- 767
3
votes
How does signal strength relate to bandwidth?
If by "strength" you mean "subjective volume as determined by a human operator", then reducing the bandwidth of the transmission while keeping the transmitter power constant does indeed produce a "...
- 52.5k
3
votes
Beam antenna changed directivity
I would suggest that one of the transmission line connections to the antenna, probably the earth if you are using coax, has become disconnected at the antenna feed point, and it's just a coincidence ...
- 3,739
3
votes
Can a common-mode current exist on the inside of a coax shield?
First, does it matter where the common-mode currents flow? Coax works as a transmission line because the (ideally) equal but opposite currents on the center and shield create opposite fields which ...
- 52.5k
3
votes
Accepted
Can you create a radio beam with a Faraday cage?
No, there would not be a narrow beam.
There's multiple ways you can show that, but most intuitively, I think is the follow:
consider every point on that patch to be a source of an elemental wave
in ...
- 17.4k
3
votes
Why can't VHF / UHF be used with ionosphere reflection?
Physics of Ionospheric Reflection
As you are probably aware, solar radiation ionizes atoms and molecules in the atmosphere at altitudes of about 50 km or so and above. The densities of free ...
- 550
3
votes
How does an antenna transform impedances along its length?
Think of the antenna as a whole as being many inductors in series along its length and along with that many capacitors branching off like a tree (capacitors in the air) going back to the "...
3
votes
How does an antenna transform impedances along its length?
The impedance along the length of the antenna is a function of the standing wave developed on the antenna. If we assume no ground losses and ignore the self capacitance at the tip of the antenna, then ...
- 18.6k
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